Satellite phage P4 and its helper P2 are an excellent model system for the in vitro and in vivo study of viral morphogenesis and gene control. P4 depends on its helper for all late gene products involved in morphogenesis. We propose to determine whether it is the P4 DNA or a P4 size directing protein which directs the assembly of the smaller P4 head from helper gene products. Satellite P4 can transactivate all of the 18 late genes of a helper P2 prophage. This occurs without excision or replication of the helper prophage genome. We propose to determine how P4 transactivates its P2 helper.